Very little is known about the Regulan eel-bird - only one reference has been made to the species canonically (whenever a Vulcan explains the Pon Farr), and there have been only a couple of references within literature.
|An Eelbird, as illustrated by the legendary Don Ivan Punchatz|
In the eBook "The Darkness Drops Again", we were introduced to a sub-species known as the pygmy eel-bird. This sub-species' feathers would reflect the UV rays of the planet's sun, so that they appeared to be a shiny purple. This may also explain the coloring in Don Ivan Punchatz's illustration (shown above) for "The Worlds of the Federation".